The present invention relates to vehicle seat belt systems and, in particular, to a pretensioner that tightens a vehicle seat belt by pulling a buckle to which the belt is attached generally downwardly from a normal upper position to a pretensioning lower position.
The seat belt systems of vehicles, such as automobiles, are often equipped with pretensioners for applying pretension to the belts when an acceleration greater than a predetermined threshold level is applied to the vehicle, such as may occur in a collision. The pretension applied to the belt tightens the belt against the occupant before he or she is thrown forward by inertial force and provides enhanced restraint of the occupant by the belt. Various types of pretensioners are well known and in common use. One general type, which is known in several specific forms, is a buckle pretensioner, which provides pretension by pulling generally downwardly on a buckle to which the belt is attached. Such belt pretensioners, when they are provided, are almost always located on the side of a seat and operate to pull down a buckle that receives a buckle tongue at the juncture between the shoulder and lap portions of a belt in a three-point belt system. In such an arrangement, pretension is applied generally equally to both the lap and shoulder portions of the belt.
Among the specific types of previously known buckle pretensioners are those that have as the drive device a piston/cylinder operated by gas pressure (see, for example, Japanese Patent Laid-open Publication No. 1-119455), or a compression coil spring mechanism (Japanese Patent Laid-open Publication No. 2-256550). In those types, the piston/cylinder or the spring mechanism, as the case may be, is connected to the buckle by a flexible drive cable, which passes over a guide roller or surface. The drive device has an operating axis that is at a sharp angle to the path of movement of the buckle, because it is necessary to have the drive device extend generally horizontally along the side of the seat. The size and position of the drive device place restrictions on the layout of the vehicle, such as the size and location of the housing of a gear shift mechanism between the seats.
Another type of previously known buckle pretensioner has as the drive device a spiral spring (Japanese Patent Laid-open Publication No. 1-164651). One end of the spiral spring is attached to the buckle support, and the other end engages and operates a clamp lever mechanism through a force-transmitting mechanism. The rotational force of the spring is converted from a rotational force to a linear or nearly linear force. An arrangement of this type places less restriction on the vehicle design than do the types that use piston/cylinders or compression springs, but the force-transmission mechanism is more complicated and has higher transmission losses.